1. Field of the Invention
The present invention relates to a reflection photomask and an exposure apparatus suitable for use in the production of large scale integrated circuits (LSIs).
2. Description of the Related Art
An optical system of a projection exposure apparatus according to the related art is shown in FIG. 12. A mirror 2 is provided between a lamp house 1 and a fly-eye lens 3. Sequentially arranged following order in the downstream in the light path from the fly eye lens 3 are: an aperture member 4; relay lenses 5A; a stop 6 between the relay lenses 5A; another mirror 7; a condenser lens 5B; and a photomask 8 having a circuit pattern. Projection lenses 9A and 9B are arranged in that order downstream from the photomask 8. A wafer 10 is placed in front of (downstream from) the projection lens 9B.
When the light emitted from the lamp house 1 is reflected by the mirror 2 and then reaches the fly-eye lens 3, the light is divided into bundles by component lenses 3a of the fly-eye lens 3. The light bundle transmitted through each of the component lenses 3a further travels via an aperture 4a of the aperture member 4, the relay lenses 5A, the stop 6, the mirror 7 and the condenser lens 5B so as to fall on the entire surface of the photomask 8. The light bundles from the component lenses 3a of the fly-eye lens 3 overlap one another on the photomask 8, thus achieving uniform illumination over the entire surface of the photomask 8. The light transmitted through the photomask 8 travels through the projection lenses 9A and 9B to strike the wafer 10. In this manner, a photoresist film provided beforehand on the surface of wafer 10 is exposed to the illumination light, thus preparing a circuit pattern.
An example of a conventionally-used photomask 8 is shown in FIG. 13. The example photomask 8 comprises: a transparent substrate 11 made of glass or the like; metallic light-blocking films 12 formed on the transparent base board 11, the light blocking films 12 forming a repetitive pattern; and phase shift members 13 selectively provided in light transmitting portions, that is, the portions not covered by the light blocking films 12. The light entering the transparent substrate 11 is varied in intensity by the light transmitting portions and the light blocking portions covered by the light blocking films 12. Further, the phase of the light transmitted through the portions covered by the phase shift members 13 is shifted by a predetermined difference from the phase of the light transmitted through other portions. The imaging characteristic is thus enhanced.
The above-described photomask 8 is produced by the steps as shown in FIGS. 14A to 14D. First, an etching stopper 14 and a light blocking film 12 are sequentially vapor-deposited on a transparent substrate 11, as shown in FIG. 14A. After the light blocking film 12 is selectively etched to form a light blocking pattern as shown in FIG. 14B, spin on glass (SOG), a material for phase shift members 13, is applied to both the etching stopper 14 and the light blocking film 12 to form a SOG film 15 as shown in FIG. 14C. Finally, the SOG film 15 is selectively etched to form phase shift members 13 as shown in FIG. 14D.
Various constructions of photomasks have been proposed. For example, a photomask shown in FIG. 15A has phase shift members 13 arranged in every other light transmitting portion of the repetitive pattern so that the light bundles transmitted through two neighboring light transmitting portions weaken each other.
A photomask shown in FIG. 15B has small auxiliary phase shifters 16 formed of SOG or the like on the peripheries of the isolated patterns. Because the auxiliary phase shifters 13 are so small that the images thereof will not be resolved, the light diffracted from the auxiliary phase shifters 13 enhances the optical image of the main pattern.
A photomask shown in FIG. 15C has phase shift members 13 on the peripheries of the openings of isolated patterns and, thereby, peripheral portions of the optical image of the main pattern are eliminated, thus enhancing the optical image.
A photomask shown in FIG. 15D has phase shift members 13 and thin light-blocking films 12 having a reduced thickness so as to achieve a half tone quality (semitransparency). The combination of the phase shift members 13 and the thin light-blocking films 12 reverses the phase of the light transmitted therethrough and, thereby, peripheral portions of the optical image of the main pattern are eliminated, thus enhancing the optical image.
Further, a photomask shown in FIG. 16A has a multi-layer phase shift member 13 having a .pi./2 phase shift portion 17 in a boundary between the 0 phase region and the .pi. phase region, thus increasing the degree of freedom in pattern layout.
A photomask shown in FIG. 16B forms light blocking portions by utilizing interference between the light phase-reversed by the phase shift members 13 and the light transmitted though portions not covered by the phase shift members 13. This photomask achieves a higher light blocking effect than other photomasks having metallic light-blocking patterns.
A photomask shown in FIG. 16C has metallic light blocking films provided in the phase shift members 13, thereby increasing the degree of freedom in pattern size.
A photomask shown in FIG. 16D forms a light blocking pattern by utilizing edge portions of relatively large phase shift members 13.
A photomask shown in FIG. 16E has such small phase shift members 13 that their images are not resolved, thus achieving a light blocking pattern having a large area without using a metallic film.
Thus, the conventional photomasks have phase shift members 13 or auxiliary phase shifters 16 formed by patterning SOG films. However, when electron beam exposure is performed, the phase shift members or auxiliary phase shifters 16 are electrically charged, thus distorting the course of electron beams. Therefore, a correction in accordance with the patterns is required.